The book metadata section most likely contains invalid refine entries.
Check the book with my
EPUBCheck plugin and fix all metadata errors.
If you don't get any error messages, try the following:
1. In Sigil, select Edit > Preferences > Open Preferences Location.
2. Go to plugins\ePub2 and open plugin.py with a text editor.
3. Change line 215 from:
Code:
if dc_property == 'role':
to
Code:
if dc_property == 'role' and dc_id is not None:
If it still doesn't work, do an epub to epub conversion with Calibre. This should convert your epub3 book to an epub2 book.